STP: force a root bridge in a redundant L2 triangle
- Switching
- Intermediate
- 12 min
- 3 steps
- Cisco IOS-style
Three switches in a triangle form a redundant L2 path. Spanning-Tree picks one as root and blocks one of the three links to break the loop. Force SW1 as root deterministically by lowering its bridge priority.
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Free · no install · no account
Scenario
Without Spanning-Tree, a triangle of switches forms a broadcast loop — a single ARP would multiply forever and saturate every link in milliseconds. STP prevents this by: 1. Electing a root bridge (lowest priority + lowest MAC wins). 2. Computing per-port roles: every non-root switch picks its best path to the root (the *root port*). Each segment then picks the side with the lowest cost-to-root as the *designated* port. Everything else becomes *alternate* — and alternate ports go to blocking state, breaking the loop. `` SW1 ── SW2 \ / \ / SW3 ` By default every Cisco switch ships with priority 32768. The tiebreaker is the MAC, which is essentially random. Real-world best practice: explicitly set the priority of your *intended* root to 4096` (one step lower than default) so an admin can predict which switch carries the root role across reboots.
Topology
3 × switch · Cisco IOS-style
SW1— switchSW2— switchSW3— switch
What you will do
Without any priority changes, STP picks a root by lowest MAC address among the three switches. With the MACs we set (aa:bb:cc:00:01:* / 02:* / 03:*), which switch will be root?
Force SW2 as the root by setting its VLAN-1 priority to 4096 (one step below the 32768 default). Real-world reason: SW2 is better-connected, has redundant power, etc.
Pin SW1 as the *secondary* root — priority 8192. If SW2 ever goes down, SW1 takes over predictably.
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This lab in other languages
STP: Root Bridge in einem redundanten L2-Dreieck erzwingenSTP: forzar un root bridge en un triángulo L2 redundanteSTP: forçar um root bridge em um triângulo L2 redundanteSTP : forcer un root bridge dans un triangle L2 redondantSTP: redundant L2 triangle में एक root bridge को force करेंSTP：在冗余的 L2 三角拓扑中强制指定 root bridgeSTP: forzare un root bridge in un triangolo L2 ridondanteSTP: 冗長な L2 三角形で root bridge を強制するSTP: 중복 L2 삼각 구조에서 root bridge를 강제로 지정합니다STP: принудительно назначить root bridge в избыточном L2-треугольникеSTP: yedekli L2 üçgeninde bir root bridge'i zorlaSTP: ép một root bridge trong tam giác L2 dư thừa